1. Field of the Invention
The present invention relates to a magnetic recording method using a microwave assisted head for writing a data signal to a magnetic recording medium having a large coercive force to stabilize a magnetization.
2. Description of the Related Art
Bit cells of digital information recorded to a magnetic recording medium are miniaturized with developments in high density recording. As a result, signals detected from a reproduction element of a magnetic head may fluctuate due to a so-called thermal fluctuation so that signal-to-noise ratio (S/N) may be deteriorated or, in the worst case, the signal may be lost.
For this reason, in a magnetic recording medium that utilizes a perpendicular recording system, which has been put to practical use in recent years, miniaturizing magnetic nanoparticles that configure a recording layer simultaneously with increasing magnetic anisotropy energy Ku that fixes the magnetization direction of the magnetic nanoparticles are effective in order to solve the above problems. A thermal stability index S that corresponds to the thermal fluctuation is expressed by S=Ku·V/kB·T, and it is said that the value of S is generally required to be not less than 50. Here, Ku is magnetic anisotropy energy, V is the volume of magnetic nanoparticles that configure the recording layer, kB is the Boltzmann constant, and T is the absolute temperature.
However, a magnetic field (magnetization reversal magnetic field) Hsw necessary for recording information is proportional to Ku, and therefore, raising Ku may cause an increase in Hsw.
In order to form magnetization reversal of the recording layer that corresponds to a preferable data series, it is required to apply a recording magnetic field with an intensity that exceeds Hsw that rapidly changes. In recent years, a recording element using a so-called single pole has been used in hard disk drives (HDD), which have made practical by the use of a perpendicular recording system, and a recording magnetic field in a perpendicular direction is applied to the recording layer from an air bearing surface (ABS).
The intensity of the perpendicular recording magnetic field is proportional to a saturation magnetic flux density Bs of a soft magnetic material that forms a single pole. Therefore, a material with a saturation magnetic flux density Bs as high as possible has been developed and put to practical use. However, the practical upper limit of the saturation magnetic flux density Bs is Bs=2.4 tesla (T) from the so-called Slater-Pauling curve, and it may be said that the current situation is approaching the practical limit. Further, the thickness and the width of the current single pole are approximately 100-200 nm. When increasing the recording density, the thickness and width need to be further reduced, and the perpendicular magnetic field generated along with the reduction has a tendency to further decrease.
From such reasons, it can be said that the recording capacity of common data writing elements is about to reach the limit, and high density recording is difficult to overcome in the current condition.
Accordingly, a so-called thermal assisted magnetic recording (TAMR) has been proposed for recording signals by irradiating the recording layer with laser beam or the like and raising its temperature to make a condition where the coercive force of the recording layer is lowered.
However, problems such as those described below still occur even in the thermal assisted recording. Namely, (1) a magnetic head equipped with a magnetic element and an optical element is essential but its structure is extremely complicated and expensive, (2) it is essential to develop a recording layer with a large variation in temperature characteristics for the coercive force, (3) thermal demagnetization in the recording process leads to adjacent track erasure and destabilization of the recording state, and the like.
In contrast, research on spin transfer in electronic conduction has been actively engaged in targeting higher sensitivity of GMR heads and TMR heads as reading elements. A research has begun for applying this to the magnetization reversal of the recording layer of the magnetic disk medium and trying to reduce the perpendicular magnetic field necessary for the magnetization reversal.
This is to apply a high frequency alternating current (AC) magnetic field into the in-plane direction of the recording medium simultaneously with the perpendicular magnetic field for recording. The frequency of the AC magnetic field to be applied into the in-plane direction is an ultra high frequency (several −40 GHz) of a microwave band that corresponds to the ferromagnetic resonant frequency of the magnetic nanoparticles that configure the magnetic recording layer (hereinafter, referred to as simply “recording layer” or “magnetic layer”) of the magnetic recording medium.
Furthermore, analysis results are reported that the magnetization reversal magnetic field Hsw of the recording layer can be decreased to inasmuch as 60% by simultaneously applying the AC magnetic field into the in-plane direction. If the present system is practical, there is no need to use the TAMR with a complicated configuration and further it becomes possible to increase Ku of the recording layer so that significant improvement of the recording density can be expected.
The phenomenon that makes the magnetization reversal magnetic field decrease can be obtained by applying the AC magnetic field with a frequency near the ferromagnetic resonance (hereinafter, occasionally referred to as “FMR”) frequency of the spin of the magnetic nanoparticles that configure the recording layer so as to excite precession movement of the magnetic nanoparticle spin.
However, since the FRM frequency of the spin sequentially varies according to the angle from a magnetization easy axis of the spin, so only with providing a sine wave of a single frequency, the effect to excite precession movement occurs only when the spin is at a specific angle in the process to achieve the magnetization reversal. An effect to excite precession movement cannot be obtained because the frequencies of the FMR and the AC magnetic field do not match at other angles of a spin.
Ideally, the optimal method is that the assisting microwave frequency sequentially varies to follow the spin angle during the precession movement; however, one cycle of the precession movement is a short cycle of 1 ns or below, and therefore, synchronizing and following in such a cycle is unrealistic.
In order to remove those disadvantages, a method has also been proposed to apply a frequency-modulated wave (hereinafter, occasionally referred to as FM wave) (JP Laid-Open Patent Application No. 2010-3339: Tohoku University). However, because this modulation is performed by a single frequency, the spectrum of the FM wave is in a state where energy is not uniform with gaps therebetween as shown in FIG. 32. It can be said that the assisting effect is extremely small when the FMR frequency of the magnetic nanoparticles that configure the recording layer enters into a gap between modulation frequencies in a missing teeth state (or comb-like state).
Further, there are some issues such as the weakening of the spectrum intensity when separating from the center frequency in the FM wave by a single frequency signal, also the energy of the center frequency may be zero in certain modulation indexes, and the like. Therefore, from these perspectives, it can be said that using the FM wave by a single frequency signal is undesirable.
The present invention is originated from such actual circumstances, and an object is to answer the demand for a proposal of a magnetic recording method that can provide a high assisting effect in which the magnetization reversal is performed efficiently by exciting the precession movement of the spin of the magnetic nanoparticles that configure the recording layer of the magnetic recording medium in both regions with lower and higher frequency than the ferromagnetic resonance (FMR) frequencies.